A type II heterostructure is defined as a structure that has one (either conduction band or valence band) band offset value equal or lower than zero while the other band offset value is larger than zero. FIG. 1 shows the schematic diagram of the type II heterostructure band lineup in comparison with type I heterostructure. For the type II structure shown in FIG. 1, valence band offset E.sub.vII,I is less than zero while conduction band offset E.sub.cII,I is larger than zero, therefore it is type II heterostructure. In another case, when E.sub.cII,I is less than zero while E.sub.vII,I is larger than zero, it is type II heterostructure as well.
For a conventional laser diode structure, where either an AlGaAs/GaAs material system or an AlGaInP/GaInAs /GaAs system is used, no type II heterostructure exists. However, in a conventional laser diode structure where both AlGaAs and AlGaInP/GaInAsP are used in a laser structure simultaneously, a type II heterostructure will exist.
Prior art laser diodes use only one single material system where the deposited layers, which define the laser, are of the same material system and the layers include the various compositions of the elements which adjusts the energy gap profile of the layers as is well under stood. Thus the familiar material systems include, for example, GaAs/AlGaAs, GaInAs/GaInP and GaInAsP/AlGaInP.
AlGaAs has been used in cladding layers as a carrier blocking layer for AlGaInAsP laser diodes (L. Mawst and D, Botez "High Power InGaAsP/InGaAIP/GaAs semiconductor diode lasers, SPIE Vol 3001, 1997, P7-P12). However, in that case, the benefit of type II band offset is not used. In other words, a higher band gap AlGaInP layer can easily fulfill the blocking function.
A carrier blocking layer is highly desirable for AlGaInP/GaInAsP laser diodes due to the intrinsic small conduction band offset of that system where a typical conduction band offset is about 30-4% of the band gap difference. Because of the small conduction band offset, electrons in the quantum wells will easily overflow to and reside in the waveguide region, and at high,h injection current, even overflow into the P-cladding region of the diode. As a result, threshold current increases, slop efficiency decreases and T0 and T1 are limited by carrier leakage.
Carrier blocking layers of a type I heterostructure can be and have been used in the waveguide region to achieve a carrier blocking function (U.S. Pat. No. 5,764,668). However, the carrier blocking layer has to be doped (e.g. 1.times.10.sup.18 cm.sup.-3) for type I heterostructure, otherwise injected carriers are blocked as well. The doping in the waveguide causes absorption of light and results in an undesirable loss of light. Although a thin blocking layer can be used (injected carriers tunnel through) without doping the layer, the overflow carriers can tunnel through the barrier as well and weaken the function of the blocking layer.
A carrier blocking layer of any type (I or II) also can be used in a cladding layer. There are no significant advantages of any type as long as the blocking barrier is high enough, because the material in the cladding layer is doped anyway. The disadvantage of the carrier blocking layer in the cladding layer is that it does not stop the carrier overflow into the waveguide region.